This Flower Stinks Like a Stressed-Out Bee

Flowers of the popular ornamental parachute plant Giant Ceropegia can make you grimace if you take a whiff, and now new research reveals what the smell mimics: stressed out honeybees being attacked by a spider or other predator.

This smell of stress would seem to be a turnoff, but the odor attracts honeybee-craving flies to the sci-fi-looking plant Ceropegia sandersonii, according to a paper published in the journal Current Biology.

As if the plant is not creepy enough, it also has "trap flowers" that keep the duped flies in their clutches until the plant gets what it wants: pollination.

"We show that trap flowers of this plant mimic alarm substances of western honeybees to lure food-stealing freeloader flies as pollinators," co-author Stefan Dötterl of the University of Salzburg said in a press release. "Flies are attracted to the flowers, expecting a meal, but instead of finding an attacked honeybee they are temporarily trapped in the non-rewarding flowers and misused as pollinators."

Photo: A honeybee eaten by a spider with food-stealing flies. A drop of venom is visible at the tip of the stinger. Credit: Gernot Kunz

Dötterl and colleagues Annemarie Heiduk and Ulrich Meve from the University of Bayreuth got the idea to investigate the flower's stinky scent after realizing that the plant was pollinated by flies from the genus Desmometopa. These flies typically feed on the drippings of honeybees that are in the clutches of a predator.

While observing a honeybee caught by a spider, they noticed that the bee extruded its sting and released a drop of venom. The bees' venom is known to contain volatile alarm pheromones, which serve to call and attract nest mates for help. The researchers began to wonder if the plant could be taking advantage of this line of communication among honeybees.

Sure enough, chemical analysis found that the flower's scent is comparable to volatiles released from honeybees when they are being attacked. The scientists also found that some of the shared compounds elicit a response in the antennae of the freeloader flies, showing the scent lures these insects.

About 4 to 6 percent of plants, including Giant Ceropegia, are pollinated by deceit. Like the parachute plant, they use false advertising by appearing to offer a reward, such as pollen or nectar, a mating partner, an egg-laying site or, in this case, the tantalizing smell of vulnerable, distressed honeybees.

The new study is among the first to describe a plant that achieves pollination by mimicking the odor of a carnivore's dinner.